The United Nations agency said a donation by Swiss drug maker Roche of enough of its Tamiflu antiviral to treat 3 million people could slow the spread of the outbreak among humans, especially in countries too poor to afford their own stockpile.

Fears of a global outbreak have risen since the avian virus spread recently from Asia into Siberia in eastern Russia and Kazakhstan, and the WHO has been urging governments to buy in antiviral drugs like Tamiflu. Officials in another Siberian region, the Altai Republic, said the virus had been found in a wild duck there, although the agriculture ministry in Moscow said its spread had been curbed.

The last major flu pandemic was in the late 1960s when some 4 million people died and health authorities say another is long overdue. Drug companies are currently working on a vaccine against H5N1, but in the meantime the WHO says antiviral drugs are the best option and recommends governments stockpile neuraminidase inhibitors -- the class of drugs to which Tamiflu belongs.

Roche has also received orders from around 30 countries, including Britain, France and Germany, for enough of the drug to cover between 20 and 40 percent of their populations. According to the WHO, only 40 countries have so far heeded its call for national contingency plans for a flu pandemic, Reuters reports.